Jonas was discussing the possibility of his disappearance with the Giver. Was the Giver upset by this discussion? Why or why not?

Before Jonas came into his life, The Giver accepted things the way they were. He knew that what the community was doing was wrong, especially after his former Receiver Rosemary committed suicide. Jonas might have gone along with the ways things had always been too if he had not had Gabriel’s influence. Although Gabriel was only a baby, he gave Jonas the opportunity to have a relationship with someone condemned to be Released.

Notice that...

Before Jonas came into his life, The Giver accepted things the way they were. He knew that what the community was doing was wrong, especially after his former Receiver Rosemary committed suicide. Jonas might have gone along with the ways things had always been too if he had not had Gabriel’s influence. Although Gabriel was only a baby, he gave Jonas the opportunity to have a relationship with someone condemned to be Released.

Notice that The Giver never makes an effort to stop Jonas’s plan. When he is harsh with Jonas after his violent reaction to the murder of the newborn twin, it is because he needs Jonas to understand that no one is evil in the community but they simply do not know better. Jonas can never look at the community the same way, and he and The Giver plan his escape with Gabriel as soon as Jonas learns Gabriel is going to be killed.

I think you can infer that The Giver even becomes a little excited about the possibility of change in the community. He agrees to stay behind and help the people when the flood of memories return, because he knows they will need him.

I agree. The Giver knew the implacations of Jonas leaving and what this would do to the community. He knew where he was needed. I almost think that it was a relief of sorts to be needed in a tangible way instead of what his life had been thus far, providing such an important service but never getting any satisfacation. The people were unaware of The Giver's power and what he provided.

The Giver was old and knew no other life than the life of the community. It would have been unreasonable for him to leave with Jonas. In addition, as he explained, he needed to stay for the community's sake. They would be deeply affected by Jonas' departure.

I don't think he was upset as much as he was clear about his role and the need for him to maintain his position in the community. i believe he was wishful that he could go, but not angry or upset with Jonas' suggestion to go.